It is standard practice to include various additives in such emulsions in order to facilitate their initial formation and to promote stability, in particular by reducing coalescence. Thus it is conventional to include one or more emulsifiers or surfactants. The use of combinations of emulsifiers of different HLB values is conventional. The use of two surfactants that are intended to react with one another is described in U.S. Pat. No. 4,472,291. The particle size in this is below 0.5 .mu.m.
Conventional emulsifiers consist of a single hydrophobic moiety and a single hydrophilic moiety (for instance an ethoxylated fatty alcohol consists of a fatty alkyl hydrophobic group and a hydroxy-terminated polyoxy ethylene hydrophilic group). However emulsifiers that have a more complex structure are known for specialised purposes.
For instance it is known from GB 2,001,083, GB 2,002,400 and EP 333501 to provide an oil-soluble emulsifier by condensing hydroxy stearic acid with itself and on to polyethylene glycol or polyethylene imine. The resultant product may contain molecules having a polyoxyethylene or polyethylene imine backbone terminated at each end by an end group containing a stearic group or a condensate of several stearic groups, thus providing terminal hydrophobes and a central hydrophilic chain. We have used these oil-soluble materials dissolved in the continuous phase of water-in-oil emulsions and suspensions.
It is also to known to use block copolymers of ethylene oxide and propylene oxide and/or butylene oxide as emulsifiers. We believe that existing emulsification systems always require, as an essential ingredient, the use of a conventional emulsifier of the type having a single hydrophobic moiety and a single hydrophilic moiety. Such emulsifiers are thought to be effective at promoting stability as a result of this single hydrophobic moiety being physically attracted into the oil phase and a single hydrophilic moiety being physically attracted into the water phase.
The emulsifiers and surfactants concentrate at the interface between the two phases, but it is also known that emulsion properties can be altered by viscosifying the water phase. Thus a water-soluble polymer that is wholly hydrophilic, for instance high molecular weight polyacrylic acid, can be distributed throughout the water phase in order to viscosity it.
It is known from, for instance, EP 126528 to provide an emulsion in oil of aqueous polymer droplets (i.e., a water-in-oil emulsion) wherein the formation of the emulsion is promoted by the use of a conventional water-in-oil emulsifier (e.g., sorbitan mono oleate) and the stability of the emulsion is promoted by an oil-soluble stabilising polymer dissolved in the continuous oil phase. This polymer can be formed by copolymerisation of water insoluble ethylenically unsaturated monomer (for instance stearyl methacrylate) with ethylenically unsaturated carboxylic acid (for instance methacrylic acid) or it can be, for instance, a polyethylene glycol-polyhydroxy stearic acid condensation product as mentioned above.
It is also known to make water-in-oil-in-water emulsions using different polymers having different solubilities. For instance an oil-soluble material described as a Polaxamer surfactant of high molecular weight is incorporated in the oil phase and a water-soluble polymer such as polyacrylic acid is incorporated in the water phase in the system described in Chemical Abstracts 105(2)11999 V.
Various other high molecular weight surfactants have been proposed in the literature for various uses, for instance in JP-a-5103969 and in EP-A-516508.
By use of conventional emulsifiers having a single hydrophile and a single hydrophobe it is often possible to make reasonably stable oil-in-water emulsions from a wide range of aqueous and hydrophobic liquids. If inadequate emulsifier is used, the heterogeneous composition will not be a stable emulsion but will instead coalesce and may break. Increasing the amount of conventional emulsifier having single hydrophobe and single hydrophile tends to result in reduced particle size and increased emulsion stability. However this reduced particle size is not always, in itself, a desirable result because the resulting rheology may then be unsatisfactory, especially when the amount of continuous phase is rather low, for instance being less (on a weight basis) than the amount of continuous water phase.
As an example, we have attempted to make an emulsion in water of a solution in oil of chlorpyrifos wherein the amount of oil phase (a solution in oil of chlorpyrifos) is more than the amount of water phase. We have found that at these high concentrations of chlorpyrifos and oil it is necessary significantly to increase the amount of conventional emulsifier to such an extent that the particle size is then very low (for instance below 0.2 .mu.m) and the resultant composition has an unacceptable rheology that is like a paste and which is difficult to mix into water.
Another disadvantage associated with increasing the amount of emulsifier is that some of the emulsifiers that would normally be considered to be suitable (for instance containing a single hydrophobe and a single hydrophilic group) are sometimes alleged to have undesirable environmental effects, for instance inadequate biodegradation or foaming properties. It is therefore desirable to minimise the amount of these.
We have observed that with conventional emulsions, on storage they have a reasonably wide particle size distribution (for instance less than 50% by weight of the particles having a size within 50% of the average particle size). Usually the distribution is bimodal, especially after storage. We have also observed that the average particle size tends to increase significantly (for instance by 50% or more) on storage even when the composition may appear storage stable, and that the particle size distribution may also increase on storage.
Oil-in-water emulsions having agricultural ingredient in the oil phase are desribed in WO89/03175. These include a surfactant in the aqueous phase and the surfactants listed are described as, for instance, ethoxylated alcohols, anionic/non-ionic blends, block copolymers, non-ionic ethoxylated alcohols and other types. It is stated that the inclusion of an aqueous latex can substantially stabilise the emulsion. It is stated that the latex combines with oil droplets of the emulsion to produce a substantial number of particles with a size between the emulsion droplets and the size of the latex particles. Equilibration between the latex particles and the emulsion droplets is said to occur, and so the stabilised emulsion presumably has the polymer of the latex in the oil droplets. Suitable polymers are said to include polystyrene, styrene butadiene polymers, styrene butyl polymers, polyvinyl acetate, vinyl acetate ethylene polymers, acrylic styrene polymers and acrylic copolymers. The oil phase into which the polymer is said to equilibrate can be selected from materials such as aromatic hydrocarbons, mineral oils, kerosene, polybutene, certain amides or esters and chlorinated hydrocarbons.
Existing systems of stabilising oil-in-water emulsions still leave room for improvement, especially when it is desired to have a relatively high amount of the emulsified oil phase in the emulsion and/or a relatively high amount of active ingredient dissolved in the oil phase. It would be desirable to be able to provide improved stabilisation of such emulsions using readily available materials and in particular materials that very cost-effective and readily available.